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He wrote: "One day she told me how matters had come to a head with the Queen Mother.

"Harry had complained that whenever he and William visited their great-grandmother at Clarence House, it was always William she made a fuss over and who sat next to her. 'It's not fair,' Harry told his mother."

Princess of Wales with her son Prince Harry (Image: PA)

Diana's closeness to both of her sons is something she has always been praised and celebrated for.

She tried hard to make their upbringing as 'normal' as possible, telling palace staff to only call them by their first names and not "prince" or "your royal highness".

A fun-loving mum, she also took them on days out to theme parks and roared with laughter as they got soaked on log flumes.

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In 2007, on the 10th anniversary of his beloved mum's death, Harry said: "She never once allowed her unfaltering love for us to go unspoken or undemonstrated. She will always be remembered for her amazing public work, but behind the media glare, to us, just two loving children, she was quite simply the best mother in the world."

William has previously spoken about the heartbreak of not having his mother there when he had his own children.

The Princess enjoyed her skiing holiday with her sons Prince William and Prince Harry in 1994. (Image: Mirrorpix)

"I would like to have had her advice," he said. "I would love her to have met Catherine and to have seen the children grow up. It makes me sad that she won’t, that they will never know her."

Her brother, Charles Spencer, said on the 21st anniversary of her death: “One of the great tragedies is that Diana would have been the best grandmother ever."

Prince Harry will undoubtedly be sharing some of those feelings after it was announced on Monday he and Meghan Markle are expecting their first child together.